1. Field of the Invention
This invention relates to a vehicle steering control system composed of front wheel operating means such as a power steering system and a rear wheel control means such as a compliance steer control system, and more particularly to an improvement in such a steering control system to selectively supply hydraulic fluid under pressure to the front wheel operating means and the rear wheel control means.
2. Description of Prior Art
In order to improve cornering performance in modern automotive vehicles, a vehicle steering control system has been proposed in which so-called compliance steer of rear wheels is controlled by a compliance steer control system under the action of hydraulic fluid to be supplied to a power steering system. Such a vehicle steering control system is disclosed, for example, in U.S. Pat. No. 4,440,254 issued on Apr. 3, 1984 in the name of Shibahata et al.
This vehicle steering control system is composed of a pressure control valve for controlling pressurized hydraulic fluid in response to the rotation of a steering wheel. The hydraulic fluid controlled by the pressure control valve is supplied to a power cylinder of a power steering system to develop a steering assist power for turning front wheels in the same direction as steering wheel rotation thereby to reduce a steering effort to be applied to the steering wheel. The controlled hydraulic fluid is also supplied to actuators of a compliance steer control system to turn rear wheels in the same direction as steering wheel rotation thereby to reduce toe-out phenomena of the rear wheels caused by side force applied to the rear wheels during vehicle cornering, thus attaining vehicle stability during high speed cornering.
Now, it will be understood that, during low vehicle speed cruising, it is necessary to supply hydraulic fluid to the power cylinder of the power steering system in order to power-assist turning front wheels against a higher resistance between the front wheels and road; however, it is not necessary to supply the hydraulic fluid to the actuators of the compliance steer control system since the side force applied to the rear wheels is smaller. Conversely, during high vehicle speed cruising, the side force applied to the rear wheels becomes greater and therefore it is necessary to supply the hydraulic fluid to the actuators thereby to decrease the toe-out phenomena due to the side force; however, the power cylinder is not necessary to be supplied with the hydraulic fluid since the driven speed of the front wheels is sufficiently high relative to its turning speed for steering, thus reducing apparent resistance between the front wheels and road.
In constrast with this, with the above-mentioned conventional vehicle steering control system, the power cylinder of the power steering system and the actuators of the compliance steer control system are unavoidably simultaneously operated because the same controlled hydraulic fluid is directly supplied to both of them. Accordingly, the actuators are unnecessarily operated at a low vehicle speed, while the power cylinder is unnecessarily operated at a high vehicle speed. As a result, a greater power is required to drive a hydraulic pump for supply the power cylinder and the actuators with pressurized hydraulic fluid, thus deteriorating fuel economy.